The UK is set for a "major constitutional crisis" in two years' time if politicians fail to give devolved governments more powers, Vincent Cable warned today.

The Lib Dem Treasury spokesman made his comments at a Guardian fringe debate on "fixing politics", as he highlighted the party's commitment to giving devolved governments greater autonomy without breaking up the UK.

Cable told a packed audience earlier today that a change of government at the next election could see the matter of greater autonomy brought to a head.

The recent Calman commission recommended greater autonomy and tax-raising powers for Scotland, but the Scottish government, led by the Scottish National party is gearing up for a referendum on independence next year, subject to the consent of the anti-independence majority in the Scottish parliament.

Cable said: "There is a scenario looming – and we hope it won't happen – but we may get a Conservative government with one or two Scottish MPs. Absolutely no mandate whatsoever north of the border.

"We have a nationalist government in Edinburgh on a collision course with London and the whole future of the UK will come into question. I believe that would be a terrible tragedy because the UK is one of the great creations of this country and something we should be proud of."

He told delegates that the party was committed to "generally federal solutions" that would let the Scots, the Welsh and the Northern Irish gain more revenue-raising powers to create "much more genuine home rule but within the UK".

"Unless we grapple with this, it will lead to conflict and possible secession. We have to start raising the warning here and now about what could happen."

Cable highlighted elements of his 10-point plan on cleaning up politics such as giving parliament more say over government spending, and reforming party funding.

"Seats will be bought" at the next election as a result of contributions from donors, some of whom are "unsavoury", said Cable.

He also called for electoral reform to stop the practice of "rotten boroughs", where MPs felt under no threat due to their large majorities. Making votes count was crucial to improving the behaviour of MPs, said Cable.

The Lib Dem Treasury spokesman, a former Glasgow councillor, also called for a return to localisation, where councils had greater control over decisions rather than having some of them dispersed to unelected quangos.

On restoring trust, fellow Lib Dem Norman Baker said greater transparency was the answer because "sunlight" was the best disinfectant. "Transparency has the impact of curbing unacceptable behaviour," he said.

Professor John Curtice, an elections expert, questioned the call to fully restore trust in British politics, because he said a bit of scepticism was "not necessarily a bad thing". "I do not necessarily want to fully restore trust and confidence in British politics."

As long as parties competed with each other to say they were purer than pure, they could not be surprised if the voters did not trust them, he added.

"They cannot be surprised that the public doesn't trust them if they do not trust each other," he said.